Method and device for jet cleaning

ABSTRACT

Blasting method for cleaning surfaces, wherein a carrier gas is supplied under pressure through a blasting line ( 10 ) to a blasting nozzle ( 14 ), and liquid CO 2  is supplied through a feed line ( 32 ), is transformed into dry snow through expansion and is fed into the blasting line ( 10 ), characterised in that CO 2  from the feed line ( 32 ) is introduced into the blasting line ( 10 ) through an expansion volume ( 34 ) having an enlarged cross section.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase of PCT International ApplicationSerial No. PCT/EP03/07011, filed Jul. 1, 2003.

FIELD OF THE INVENTION

The invention relates to a blasting method for cleaning surfaces,wherein a carrier gas is supplied under pressure through a blasting lineto a blasting nozzle, and liquid CO₂ is supplied through a feed line, istransformed into dry snow by expansion and is fed into the blastingline, as well as an apparatus for carrying out this method.

DESCRIPTION OF THE BACKGROUND ART

A blasting method of this type has been disclosed in U.S. Pat. No.5,616,067 A. The CO₂ is introduced in liquid form into an annularchamber which surrounds the blasting line through which compressed airis passed, and from there the CO₂ is fed into the blasting line througha circular array of converging capillaries, so that the expansion occursonly upon entry into the blasting line. The dry snow thus created isentrained and accelerated by the compressed air and is jetted onto theworkpiece to be cleaned via the blasting nozzle. This method isparticularly intended for gently cleaning pressure-sensitive surfaces insuch as electronic circuit boards.

U.S. Pat. No. 5,679,062 describes a blasting method in which gaseous orliquid CO₂ or a mixture of gas and liquid is expanded at the outlet of anozzle and is introduced into an enlarged vortex chamber in which a partof the gaseous and/or liquid CO₂ is transformed into dry snow. Theoutlet of the vortex chamber is directly coupled to the blasting nozzle.Here, the carrier gas is formed by the gaseous CO₂ that has beensupplied or is produced through evaporation.

U.S. Pat. No. 5,725,154 A describes a blasting method in which dry snowis produced by expanding liquid CO₂ by means of an expansion valve.Through a thin tube which is coaxially surrounded by a tube forsupplying the carrier gas, the dry snow is supplied to a blasting pistolwhich then jets out in a mixture of carrier gas and dry snow.

WO 00/74 897 A1 discloses a blasting apparatus in which liquid CO₂ issupplied via a capillary which opens into a conically divergent nozzlethe diameter of which increases towards the outlet to approximatelythree times the diameter of the capillary. This nozzle is surrounded byan annular Laval nozzle in which the carrier gas that has been suppliedunder pressure is accelerated to supersonic speed. The mouths of the CO₂nozzle and the Laval nozzle are level with one another, so that twoconcentric jets are produced, i.e. an inner jet consisting mainly of dryice and a jacket jet which is to accelerate the dry ice outside of thenozzle.

Also in applications in which larger surfaces such as the internalsurfaces of pipes or boilers in industrial equipment shall be freed offirmly adhering incrustations, the use of dry ice or dry snow asblasting material, depending on the type of incrustations, is frequentlydesirable, because the low temperature of the dry ice or dry snow makesthe material to be removed more brittle. When particles of dry snowpenetrate into the layer to be removed whith sufficient kinetic energy,a cleaning effect is achieved by the fact that the particles of drysnow, when penetrating into the layer to be removed, are evaporatedabruptly and thus blow off parts of the layer to be removed. Anotheradvantage is that no additional means are necessary for discharging theused blasting material, because the dry snow evaporates to gaseous CO2.

However, the blasting methods described above are not suitable for thesekinds of application, because the achievable volume flow rates and jetspeeds are not sufficient and/or because dry snow is not produced in asufficient amount or does not have the correct composition, so that thekinetic energy of the particles of dry snow is to small.

For this reason, for cleaning larger, heavily contaminated surfaces,blasting equipments have heretofore been used in which dry ice or drysnow is stored in solid form in suitable cooling tanks and is meteredinto the flow of compressed air. The compressed air and the dry snowserving as blasting material are then delivered together through apressure hose which connects the blasting equipment to the blastingnozzle. However blasting methods and apparatus of this type requirecumbersome installations and correspondingly high equipment costs aswell as high expenses for storing the dry snow.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide blasting methodsand blasting apparatus in which high blasting powers and high cleaningeffects can be achieved with little effort.

This object is achieved with the features disclosed herein.

According to the invention, in a method of the type indicated in theopening paragraph, the CO2 is supplied from the feed line into theblasting line via an enlarged expansion volume.

Surprisingly, it has been shown that, by suitably dimensioning theexpansion volume and/or by suitably conducting the method, it ispossible to create large amounts of dry snow having a high cleaningeffect. In particular, it is possible with this method to achieve highflow rates of 0.75 to 10 m³/min or more, so that even larger or heavilycontaminated surfaces can be cleaned efficiently. Since the dry snowserving as the blasting material is created from liquid CO2 only at thetime when the blasting method is practised, it is possible to save thehigh costs for the blasting equipment and for storing the dry snow,which have heretofore been necessary.

According to one embodiment, the production of strongly abrasive drysnow or dry ice is achieved simply by providing an expansion volume withsufficiently large volume. In experiments it was possible to multiplythe cleaning effect by increasing the expansion volume, when the otherconditions were left unchanged. This surprising phenomenon is presumablydue to the fact that the larger expansion volume between the mouth ofthe feed line and the point of entry into the blasting line leads to atemporary reduction of the flow velocity and hence to an increasedparticle density, so that the finely dispersed dry snow particles thatare at first created upon expansion agglomerate or condense to largerparticles before they are entrained by the flow of the carrier gas. Thisleads to the production of snow particles which have a larger mass andthen produce a high cleaning effect because of their higher kineticenergy.

For the volume V of the expansion volume in relation to thecross-sectional area A of the feed line for the liquid CO² the followingrelation should be observed:V ^(1/3) /A ^(1/2)>3 or preferably V ^(1/3) /A ^(1/2)>10.

Alternatively, the volume V of the expansion volume may be given inrelation to a flow rate φ of liquid CO₂. In this case, the relationwhich should be observed is:V/φ>0.2 m³ s/kg, preferably V/φ>0.6 m³ s/kg.

The method may also be practised with a smaller volume of the expansionvolume, if the smaller volume is compensated for by a larger pressureand a correspondingly increased flow rate of the carrier gas and/or ifthe expansion volume has a sufficient length, for example a length atleast 15 or 30 mm.

The temperature prevailing in the expansion volume is considered to bean important factor for the production of strongly abrasive particles ofdry ice. This temperature should preferably be low, preferably below−40° C. When the method according to the invention is practised with asufficient flow rate of carrier gas (e.g. 0.75 m³/min) and when the flowrate of liquid CO₂ is in an optimal ratio to the flow rate of air, e.g.in the order of magnitude of 0.1 to 0.4 kg CO₂ per m³ carrier gas(volume under atmospheric pressure), the cooling effect caused by theevaporation of CO₂ appears to be so large that the expansion volume iskept on a sufficiently low temperature.

A good thermal insulation of the expansion volume permits to exploit thecooling effect more efficiently and thereby to achieve the even lowertemperatures in the expansion volume and/or to reduce the expansionvolume. Thus, according to another embodiment of the method, anexpansion volume is thermally insulated from the environment, so thatthe desired high cleaning effect can also be achieved with a smallvolume of the expansion volume and small flow rates. Here, it has beenfound to be advantageous that the feed line for liquid CO2 is alsothermally insulated from the environment and has a good thermal contactwith the walls of the expansion volume (e.g. by means of a heatexchanger), so that the liquid CO2 is pre-cooled already to some extentin the feed line.

It has been found in experiments that a relatively strong crust of dryice is deposited already after a short time of operation on the walls ofthe expansion volume and/or the walls of the blasting line, and thecrust may even extend into the blasting nozzle. This crust of dry iceimproves the thermal insulation and cooling of the expansion volume andmay also contribute directly to the creation of relatively coarse andhard particles of dry ice having a high cleaning effect. When the drysnow which is first produced by expanding the liquid CO2 is swirled, itimpinges onto the walls of the expansion volume and/or the blasting linewith high velocity, so that the above-mentioned, relatively strong andcondensed crust is built-up there. On the other hand, the supply of heatvia the walls of the expansion volume and the blasting line and thesublimation of CO2 caused thereby tends to loosen the crust. Thus, thecrust finally assumes an inhomogeneous, granulated and relativelybrittle structure, with the result that the carrier gas passing-by withhigh speed permanently erodes coarse dry ice particles from the crust,and these particles then form part of the blasting material.

The desired production of such a crust of dry ice can be brought aboutor assisted by the presence of swirl edges in the flow path and by theswirling of the dry snow caused thereby. Thus, according to anotherembodiment of the invention, the blasting apparatus has at least oneswirl edge in the flow path between the mouth of the feed line for theliquid CO2 and the blasting nozzle. This swirl edge may for example beformed at the transition point between the expansion volume and theblasting line, when the expansion volume opens laterally into theblasting line. Moreover, such swirl edges may also be formed by aninternal threading in a pipe section forming the expansion volume or bystationary or moveable internal structures such as a propeller wheel, aworm or the like in the expansion volume.

Suitable for executing the method is also a blasting apparatus having asource of liquid CO2, an expansion nozzle connected to said source forgenerating dry snow, and a blasting nozzle connected to a pressuresource and converging towards a constriction an diverging from theconstriction for accelerating the dry snow, wherein the expansion nozzleis arranged upstream of the constriction.

Useful detailed and further developments of the invention are indicatedin the dependent claims.

It has been found to be advantageous when the expansion volume entersinto the straight blasting line at an angle of about 10 to 90°,preferably 20 to 45° in the flow direction. With this configuration, theflow of the carrier gas produces a certain drag, and the dry snow isgently deflected into the flow direction in the blasting line. Since theflow of the carrier gas in the blasting line has a component transverseto the longitudinal direction of the expansion volume, it is to beexpected that a vortex is created at least in the downstream portion ofthe expansion volume, which vortex prolongs the dwell time of the drysnow in the expansion volume and hence the agglomeration and the growth,respectively, of the particles and the crust, respectively, of dry ice.When the diameter of the blasting line is small, the angle of entry ispreferably more acute in order to prevent the dry ice from impingingonto to the opposing wall of the blasting line.

In a suitable embodiment, the point of entry of the expansion volumeinto the blasting line is located in a small distance upstream of theblasting nozzle.

The blasting nozzle preferably has a constriction, so that the carriergas and the blasting material are accelerated to high speed.

Particularly preferred is the configuration of the blasting nozzle as aLaval nozzle in which an acceleration to approximately sonic speed orsupersonic speed is achieved. The distance between the point of entry ofthe expansion volume into the blasting line and the constriction of theblasting nozzle should preferably be larger than the diameter of theblasting line.

When dimensioning the Laval nozzle, it should be taken into account thatthe supply of dry ice immediately upstream of the nozzle reduces thetemperature of the medium and increases the density thereof, whichcauses a shift in the working point of the Laval nozzle. In order toachieve an optimal cleaning effect, in the method according to theinvention, the cross-sectional area of the constriction of the Lavalnozzle should be selected larger than it would be selected in the casethat the medium is supplied with like pressure and like flow rate onlyvia the blasting line. Moreover, the sublimation of dry snow increasesthe gas volume and leads to an acceleration of the flow of gas before,in or behind the constriction of the nozzle. Depending on the pressureconditions, droplets of liquid CO2 may also enter into the blasting lineor the blasting nozzle and evaporate there. By regulating the flow ofcarrier gas, the position where this evaporation and/or sublimationtakes place can be adjusted such that an optimal jet speed is achieved.

When the flow rate of the carrier gas is too large so that a highdynamic pressure is built up in front of the blasting nozzle, the amountand the cleaning effect of the generated dry snow is reduced. It istherefore convenient to provide a metering valve in the blasting lineupstream of the point of entry of the expansion volume, for optimallyadjusting the flow rate of the carrier gas. Preferably, another meteringvalve is provided in the feed line for liquid CO2 immediately at thepoint of entry into the blasting apparatus, so that the ratio of flowrates of carrier gas and CO2 may be adjusted immediately at the blastingapparatus.

All the measures discussed above may suitably be combined with oneanother.

In a useful further development of the method, a small amount of wateror of another solid or liquid blasting material (e.g. solid dry icepellets) is injected into the flow of carrier gas and/or into theexpansion volume in order to further enhance the cleaning effect.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiment examples will now be explained in conjunction with thedrawings, in which:

FIG. 1 shows a sectional view of a blasting apparatus for carrying outthe method according to the invention;

FIG. 2 is a sectional view of a blasting apparatus according to modifiedembodiment;

FIG. 3 shows an enlarged detail of FIG. 2;

FIG. 4 is a schematic sectional view of a blasting line which tapersstep-wise, and

FIGS. 5 to 7 show sectional and front views of a nozzle of the blastingapparatus.

As is shown in FIG. 1, a blasting line 10 is formed by a straightcylindrical pipe which has an internal diameter DL of 39 mm. An inletport 12 of the blasting line is connected to a compressor which has notbeen shown and from which compressed air is supplied with a pressure of1.1 MPa, for example. The blasting nozzle 14 configured as a Lavalnozzle is coupled to the mouth of the blasting line 10. The blastingnozzle has a convergent section 16 the internal diameter of whichdecreases from 32 mm at the upstream end to 12.5 mm at a constriction18, and a divergent section 20 the internal diameter of which increasesfrom the constriction 18 to 19 mm at the downstream end. The totallength LL of the blasting nozzle is 224 mm. The length LC of theconverging section 16 is 83 mm.

A connecting sleeve 22 between the blasting line 10 and the Laval nozzle14 has an internal diameter of approximately 32 mm, corresponding to theupstream diameter of the blasting nozzle.

Immediately upstream of the connecting sleeve 22 the pipe forming theblasting line 10 has a branch 24 which enters into the blasting line 10at an angle of 45° in flow direction. The distance D between the branch24 and the upstream end of the blasting nozzle 14 is approximately 66mm. A metering valve 26, a ball valve for example, is arranged in theblasting line 10 upstream of the branch 24. A tubular transition piece28 is screwed into the branch 24, and the upstream end of the transitionpiece is connected to a flexible feed line 32 for liquid CO2 through anadapter 30.

The feed line 32 is connected to a pressure bottle, which has not beenshown and which accommodates an amount of CO2 under such a pressure thatthe CO2 remains liquid at environmental temperature. This pressureamounts to approximately 5.5 MPa, for example, for an environmentaltemperature of 20° C. The feed line 32 has an internal diameter of 3 mm.The liquid CO2 exits through the feed line 32 due to the differentialpressure, without any need for active displacement means. The flow rateis limited by the small cross section of the feed line 32.

The transition piece 28 forms an expansion volume 34 which has twosections 36, 38 with different diameters. The upstream section 36directly adjacent to the feed line 32 has an internal diameter DC 1 of20 mm and a length L1 of 85 mm. The downstream section 38 adjoins via ashort conical section and has an internal diameter DC2 of 32 mm and alength L2 of 105 mm. The total length LE of the expansion volume 34 isthus 190 mm. The branch 24 has an internal diameter DC3 of 39 mm,identical with the internal diameter DL of the blasting line 10.

At the point in the adapter 30 where the feed line 32 opens into theexpansion volume 34, the liquid CO2 can expand abruptly. This causes apart of the CO2 to be evaporated. The evaporation and decompressionleads to a reduction in temperature, so that another part of the liquidCO2, which is finely dispersed at entry into the expansion volume,condenses to fine particles of dry snow. Since the cross-sectional areaof the upstream section 36 of the expansion volume 34 is approximately44 times the cross-sectional area of the feed line 32, the mixture ofgaseous CO2 and dry snow passes through the upstream section 36 of theexpansion volume at moderate speed. At entry into the downstream section38 the speed is reduced further. During the travel through thecomparatively long expansion volume 34 the fine particles of dry snowmay aggregate to larger particles (agglomeration). Since the flowvelocity decreases upon entry into the downstream section 38 and,accordingly, the dynamic pressure increases, the particles may also growto some extent through re-condensation of gaseous CO2. Thus, at entryinto the still larger branch 24, relatively large dry snow particleshave formed, which are now sucked by the drag of the compressed airpassing through the blasting line 10 and are entrained towards theblasting nozzle 14. In the blasting nozzle 14, the compressed air andthe dry snow are accelerated to high speed, possibly to supersonicspeed, so that a jet with high cleaning efficiency exits from theblasting nozzle. When this jet impinges on a surface to be cleaned, thedry snow acts as a blasting material for efficiently cleaning thesurface.

Experiments have shown that the cleaning effect of the jet that has beengenerated in this way depends on the dimension of the expansion volume34 and on the flow rate of compressed air in the blasting line 10.Without expansion volume, the cleaning effect is significantly reduced.Likewise, the cleaning effect is reduced dramatically when the flow rateof compressed air in the blasting line 10 is too large. For this reason,the flow rate is so adjusted by means of the metering valve 26 that anoptimal production of dry snow and an optimal cleaning effect areachieved.

The embodiment example described above may be modified in various ways.

It is possible for example to use an angled blasting line instead of thestraight blasting line 10, so that the expansion volume and the upstreamsection of the blasting line merge symmetrically into the downstreamsection of the blasting line. An arrangement in which the blasting line10 is enlarged to an annular space which coaxially accommodates theexpansion volume, is also conceivable.

In another embodiment, a hose section of considerable length may beprovided between the point where the expansion volume opens into theblasting line, and the blasting nozzle 14.

In order to produce increased amounts of dry snow, it is possible tohave a plurality of feed lines 32 entering into the blasting line 10 viarespective expansion volumes. The points of entry of the expansionvolumes into the blasting line may be distributed over the periphery ofthe blasting line and/or may be offset in axial direction. It is alsopossible to have a plurality of feed lines 32 opening into a commonexpansion volume.

Instead of compressed air, another carrier gas may be supplied via theblasting line 10. Another blasting material may be added to this carriergas or to the compressed air. Likewise is it conceivable to haveadditional solid or liquid blasting media entering into the blastingline via lateral feed lines upstream or downstream of the branch 24 orpossibly also into the expansion volume 34.

FIG. 2 shows a blasting apparatus according to a modified embodiment.Here, the expansion volume 34 is formed only by the interior of thebranch 24. This branch has an internal threading 40 into which theadapter 30 has been screwed-in. A metering valve 42 is provided in thefeed line 32 at a small distance upstream of the adapter 30, so that theflow rate of liquid CO2 may be adjusted. A flow rate of liquid CO2 ofapproximately 0.1 to 0.3 kg per m³ carrier gas (air) has proved to be afavourable setting (the flow rate of carrier gas is given as the volumeof carrier gas under atmospheric pressure).

The portion of the blasting line 10 which includes the branch 24, andthe portion of the feed line 32 directly adjacent to the adapter 30 areembedded in a sheath of thermally insulating material which has beenshown in dotted lines in the drawing. This facilitates not only thehandling of the pistol-type blasting apparatus but also improves thethermal insulation of the expansion volume 34 and the portion of thefeed line adjacent thereto, so that a low temperature in the expansionvolume is achieved.

In FIG. 3, the branch 24 has been shown in an enlarged scale. It can beseen that the internal threading 40 extends beyond the adapter 30 andforms a part of the internal wall of the expansion volume 34. The flowpath for the dry snow from the mouth of the feed line 32 into theblasting line 10 is limited by a number of swirl edges. A first swirledge is formed directly by the abrupt increase in cross section from thefeed line 32 to the internal cross section of the expansion volume 34 atthe internal surface of the adapter 30. Other swirl edges are found atthe point of entry of the branch 24 into the blasting line 10. Moreover,the grooves of the internal threading 40 also act as swirl edges. Theseswirl edges cause the dry snow forming in the expansion volume 34 toswirl, and especially the internal threading 40 promotes the adhesion ofdry snow at the walls of the branch 24, so that a relatively compact butbrittle crust 46 of dry ice is formed in the expansion volume and tosome extent also in the blasting line 10. The CO2 which is spayed out ofthe feed line 32 and is evaporated thereby forces its way through thecrust of dry ice. This CO2 and the carrier gas flowing at high speedthrough the blasting line 10 and past the crust 46 permanently erodesmall particles of dry ice from the crust. These relatively coarse andhard particles then form a very efficient blasting material by which ahigh cleaning effect of the blasting apparatus is achieved. Theparticles of dry ice may even grow further on their way through theblasting nozzle 14, because they are swept and accelerated by thecarrier gas which contains finer particles of dry snow. The exactlocation where the agglomeration of dry ice and the formation of thecrust 46 takes place depends on the specific conditions and may shift(in both directions) more or less into the blasting line 10 and possiblyinto the blasting nozzle 14.

In the example shown, the expansion volume 34 has the same internaldiameter as the blasting line 10, it may however has have a smallerinternal diameter, if desired. The angle at which the branch 24 mergesinto the blasting line 10 may also be varied, preferably in a rangebetween 20 and 45°.

In the example shown in FIG. 2 the length LE of the expansion volume(measured on the central axis) is approximately 49 mm, and the diameterDC3 of the expansion volume is 32 mm. Then, the expansion volume 34 hasa volume V of approximately 39 cm³. When the feed line 32 has aninternal cross-sectional area of 7 mm², corresponding to a diameter of 3mm, the ratio V1/3/A1/2 is approximately 12.8. In practise, the air flowrate through the blasting line 10 is preferably between 3 and 10 m³/min,with an optimum at about 5.5 m³/min. For a CO2/air ratio of 0.3 kg/m³,the corresponding flow rates j of CO2 are approximately 0.0015 kg/s to0.05 kg/m³ and 0.023 kg/s, respectively, for the optimum. Thecorresponding values for the ratio V/j are then 0.0026-0.0008 m³ s/kgand 0.0018 m³ s/kg for the optimum. The constriction 18 of the blastingnozzle 14 has a diameter of 13.1 mm.

In another embodiment, which has not been shown, the blasting line 10has a smaller internal diameter of 12.7 mm, the diameter DC3 of theexpansion volume 34 is also 12.7 mm, and the length LE of the expansionvolume is approximately 37 mm. In this case, the expansion volume has avolume V of about 4.7 cm³. The air flow rate is then preferably between1.5 and 2.5 m³/min. When the CO2/air ratio is again 0.3 kg/m³, oneobtains a value between 0.00062 and 0.00037 m³ s/kg for the ratio V/j.The value of V1/3/A^(1/2) is in this case approximately 6.3. In thiscase the constriction 18 of the blasting nozzle 14 preferably has adiameter of 8 mm.

Under these conditions, supersonic speed can be reached downstream ofthe blasting nozzle 14.

It is convenient to provide a baffle at the mouth of the blasting nozzlein order to reduce the generation of noise.

Whereas, in the examples described above, the internal cross section ofthe blasting line remains essentially constant, embodiments are possiblein which this internal cross section varies. For example, the internalcross section of the blasting line may be reduced in two steps, withsmooth transitions, as is shown in FIG. 4. Possible positions for thebranch 24 have also been shown in FIG. 4.

As will be understood from the examples given above, the expansionvolume should not be too small and, in particular, should not have a toosmall length. In an embodiment which is presently considered to bepreferable, the length of the expansion volume is 100 mm or more.

Whereas the feed line 32 has an internal diameter of 3 mm in the shownembodiments, other embodiments are possible, in which the feed line 32upstream of the expansion volume 34 or preferably at the point of entryinto the expansion volume has a diameter of only 1.0 or 1.3 mm.

For supplying liquid CO2 via the feed line 32, optionally, a cold tankmay be provided in which the CO2 is kept liquid at a temperature ofapproximately −20° C. and at a pressure of less than 2.2 MPa, e.g. 1.8MPa.

FIGS. 5 to 7 show a modified embodiment of the blasting nozzle 14, whichhas the function of a Laval nozzle but is configured as a flat nozzleand permits to create a fan-like divergent jet having a relativelyuniform density and velocity profile over its width. This blastingnozzle has, on the upstream end, a cylindrical portion 14 a with alength La and an internal diameter Da, which is adjoined by a transitionpiece 14 b with the length Lb. Adjoining on the downstream side is aflattened section 14 c with a length Lc and a rectangular internal crosssection. The transition piece 14 b serves for adapting the cylindricalinternal cross section of the section 14 a to the rectangular internalcross section of the section 14 c. This rectangular internal crosssection has an essentially constant with W and a height which increasesfrom a value H 1 at the constriction, at the end of the transition piece14 b to a somewhat larger value H2 at the mouth. In this way, anincrease in cross-sectional area in accordance with the principle of aLaval nozzle is achieved, although the width W is practically constant.If at all, the width W may increase slightly in the vicinity of themouth.

In a practical embodiment, the blasting nozzle 14 according to FIGS. 5to 7 has the following dimensions: La = 55 mm Lb = 55 mm Lc = 130 mm Da= 27 mm W = 45 mm H1 = 3.0-4.0 mm H2 = 7, 5 mm

The following dimensions apply to another embodiment example: La = 34 mmLb = 76 mm Lc = 130 mm Da = 12 mm W = 16 mm H1 = 2.25-2.60 mm H2 = 3.75mm.

The internal surface in the flattened section 14 c has corrugationswhich, in the example shown, are formed by longitudinal ribs 14 b. Suchcorrugations lead to a significant reduction of noise, especially in thesupersonic mode.

1. Blasting method for cleaning surfaces, wherein a carrier gas issupplied under pressure through a blasting line (10) to a blastingnozzle (14), and liquid CO₂ is supplied via a feed line (32), istransformed into dry show by expansion and is fed into the blasting line(10), characterised in that the CO₂ is introduced from the feed line(32) into the blasting line (10) via an expansion volume (32) having anenlarged cross section, and the volume V of the expansion volume and theinternal cross-sectional area A of the feed line (32) fulfill therelation V^(1/3)/A^(1/2)>3.
 2. Blasting method according to claim 1,characterised in that the volume V if the expansion volume and theinternal cross-sectional area A of the feed line (32) fulfill therelation V^(1/3)/A^(1/2)>10.
 3. Blasting method for cleaning surfaces,wherein a carrier gas is supplied under pressure through a blasting line(10) to a blasting nozzle (14), and liquid CO₂ is supplied via a feedline (32), is transformed into dry show by expansion and is fed into theblasting line (10), characterised in that the CO₂ is introduced from thefeed line (32) into the blasting line (10) via an expansion volume (32)having an enlarged cross section, and the flow rate ratio between CO₂and carrier gas is at least 0.1 kg/m³ preferably at least 0.25 kg/m³. 4.Blasting method for cleaning surfaces, according to claim 3, wherein acarrier gas is supplied under pressure through a blasting line (10) to ablasting nozzle (14), and liquid CO₂ is supplied through a feed line(32), is transformed into dry snow by expansion and is fed into theblasting line (10), characterised in that the CO₂ from the feed line(32) is introduced into the blasting line (10) via an expansion volume(34) having an enlarged cross section, and the ratio between the volumeV of the expansion volume (34) and the flow rate of CO₂ amounts to atleast 0.0002 m³ s/kg.
 5. Blasting method for cleaning surfaces, whereina carrier gas is supplied under pressure through a blasting line (10) toa blasting nozzle (14), and liquid CO₂ is supplied via a feed line (32),is transformed into dry snow by expansion and is fed into the blastingline (10), characterised in that the CO₂ is introduced from the feedline (32) into the blasting line (10) via an expansion volume (32)having an enlarged cross section, and in that the expansion volume (34)is thermally insulated from the environment.
 6. Blasting methodaccording to claim 5, characterised in that the portion of the feed line(32) adjacent to the expansion volume (34) is also thermally insulatedfrom the environment.
 7. Blasting method for cleaning surfaces, whereina carrier gas is supplied under pressure through a blasting line (10) toa blasting nozzle (14), and liquid CO₂ is supplied via a feed line (32),is transformed into dry snow by expansion and is fed into the blastingline (10), characterised in that the CO₂ is introduced from the feedline (32) into the blasting line (10) via an expansion volume (32)having an enlarged cross section, and in that a deposition of solid dryice at the walls of the expansion volume (34) and/or the blasting line(10) is promoted by swirl edges (40) in the expansion volume or at thedownstream end thereof.
 8. Blasting method for cleaning surfaces,according to claim 1, wherein a carrier gas is supplied under pressurethrough a blasting line (10) to a blasting nozzle (14), and liquid CO₂is supplied via a feed line (32), is transformed into dry snow throughexpansion and is fed into the blasting line (10) and discharged througha blasting nozzle (14) having a constriction (18), characterised in thatthe CO₂ from the feed line (32) is introduced into the blasting line(10) via an expansion volume (34) having an enlarged cross section, sothat a mixture of gaseous, liquid an solid CO₂ is produced in theexpansion volume and a part of the solid and liquid components evaporatein the blasting line or the blasting nozzle, and in that the position ofthe evaporation zone relative to the constriction (18) is determined byregulating the flow of carrier gas.
 9. Method according to claim 1,characterised in that the flow of carrier gas is throttled by means of ametering valve (26) upstream of the point of entry of the expansionvolume (34) into the blasting line (10).
 10. Method according to claim9, characterised in that the carrier gas is supplied to the meteringvalve (26) with a pressure of at least 0.1 MPa, preferably about 1.0 to2.0 MPa.
 11. Method according to claim 1, characterised in that the CO₂is supplied via the feed line (32) at environmental temperature andunder a pressure necessary for maintaining the liquid state.
 12. Methodaccording to claim 1, characterised in that the CO₂ is supplied throughthe feed line (32) at a temperature of less than −15° C. and at apressure necessary for maintaining the liquid state.
 13. Methodaccording to claim 1, characterised in that the mixture of carrier gasand dry snow is accelerated in the blasting nozzle (14) to at leastapproximately sonic speed.
 14. Method according to claim 1,characterised in that the expansion volume (14) has a length of at least15 mm, preferably at least 49 mm.
 15. Apparatus for carrying out themethod according to claim 1, having a blasting line (10) for supplying acarrier gas and a feed line (32) for liquid CO₂, characterised in thatthe feed line (32) is connected to the blasting line (10) through anexpansion volume (34), and the volume V of the expansion volume and theinternal cross-sectional area A of the feed line (32) fulfil therelationV ^(1/3) /A ^(1/2)>3.
 16. Apparatus according to claim 15, characterisedin that the cross section of the expansion volume (34) increases fromthe feed line (32) towards the blasting line (10).
 17. Apparatus forcarrying out the method according to claim 1, comprising a blasting line(10) for supplying a carrier gas and a feed line (32) for liquid CO₂,characterised in that the feed line (32) is connected to the blastingline (10) through an expansion volume (34), and in that at least oneswirl edge (40) is formed in the expansion volume (34) and/or at thetransition between the expansion volume (34) and the interior of theblasting line (10).
 18. Apparatus for carrying out the method accordingto claim 1, comprising a blasting line (10) for supplying a carrier gasand a feed line (32) for liquid CO₂, characterised in that the feed line(32) is connected to the blasting line (10) through an expansion volume(34), and in that at least the expansion volume (34) is surrounded by athermally insulating sheath (44).
 19. Apparatus according to claim 15,characterised in that the internal cross section of a downstream section(38) of the expansion volume (34) is approximately equal to the internalcross section of the blasting line (10).
 20. Apparatus according toclaim 15, characterised in that the expansion volume (34) enters into astraight section of the blasting line (10) from one side.
 21. Apparatusaccording to claim 19, characterised in that the expansion volume (34)enters into the blasting line (10) at an angle from 5 to 90° in flowdirection.
 22. Apparatus according to claim 15, characterised in thatthe expansion volume (34) has a length of at least 15 mm, preferably atleast 49 mm.
 23. Apparatus according to claim 15, characterised in thata convergent/divergent nozzle, preferably a Laval nozzle, is connectedas a blasting nozzle (14) to the downstream end of the blasting line(10).
 24. Apparatus according to claim 23, characterised in that theinternal diameter of the blasting nozzle (14) at its inlet opening isapproximately equal to the internal diameter of the blasting line (10),and in that the internal diameter of a constriction (18) of the blastingnozzle is approximately 15 to 75%, preferably about 35 to 45% of thediameter at the inlet opening.
 25. Apparatus according to claim 23,characterised in that the distance between the point of entry of theexpansion volume (34) into the blasting line (10) and the constriction(18) of the blasting nozzle (14) is larger than the diameter (DL) of theblasting line (10).
 26. Apparatus according to claim 15, characterisedin that a metering valve (26) is arranged in the blasting line (10)upstream of the point of entry of the expansion volume (34). 27.Apparatus according to claim 15, characterised in that a metering valve(42) is arranged in the feed line (32) directly upstream of theexpansion volume (34).
 28. Apparatus for carrying out the methodaccording to claim 1, comprising a blasting line (10) for supplying acarrier gas and a feed line (32) for liquid CO₂, characterised in thatthe feed line (32) is connected to the blasting line (10) through anexpansion volume (34) the length of which amounts to at least 15 mm,preferably at least 30 mm.
 29. Apparatus for carrying out the methodaccording to claim 1, comprising a source (40) for liquid CO₂, anexpansion nozzle (32) connected to said source, for generating dry snow,and a blasting nozzle (14) connected to a pressure source and convergingtowards a constriction (18) and diverging from said constriction foraccelerating the dry snow, characterised in that the expansion nozzle(32) is arranged upstream of the constriction (18) of the blastingnozzle (14).
 30. Blasting apparatus according to claim 1, characterisedthat the blasting nozzle (14) is a flat nozzle, having a cylindricalsection (14 a), a transition piece (14 b) and a flattened section (14c), the flattened section having an approximately rectangular internalcross section.